The invention relates to an actuation device to actuate a clamping mechanism to clamp objects such as semiconductor substrates. The clamp may be used in an indexing system of a wire bonding machine, in particular a gold wire bonding machine forming electrical connections between semiconductor dies and leadframes, but is capable of wider application.
In back-end semiconductor assembly processes, thermosonic ball bonding is by far the most demanding. To make an interconnect between a semiconductor device (commonly called xe2x80x9cdiexe2x80x9d) and its substrate (xe2x80x9cleadframexe2x80x9d), very thin wire, commonly gold wire of between 20 to 75 microns in diameter, is bonded, first onto a pad on the die and then onto the corresponding lead of the lead-frame on which the die is mounted. This process continues until all the pads and corresponding leads are connected. One leadframe may have anywhere from less than ten to a few tens of dies. In order to bring successive dies into a bonding area, a linear indexing system (xe2x80x9cindexerxe2x80x9d), is often used to index the leadframe with high precision. The indexer may essentially comprise a horizontal linear motor driven servo controlled stage on which are mounted two index clamps, one each for indexing leadframes near the input and output sections of a wire bonding machine respectively. Each index clamp has an actuator to control the clamping and de-clamping of the leadframe.
As requirements of both speed and accuracy of wire bonding grow more stringent, all critical subsystems of the wire bonding machine, including the indexer and the index clamp need to be made increasingly more robust, reliable and accurate. For the index clamp, this translates to a requirement of higher clamping force without undue increase in clamp mass. A clamping force of 40N may be required for the clamp for which the cycle time can vary anywhere from 2 to 60 seconds, depending upon the die size which in turn determines the number and length of the wires to be bonded. Correspondingly, the duty cycle can vary widely from 10 to 80%. This is so because at times, the index clamp needs to remain clamped near an already bonded semiconductor package even as bonding takes place on another die which is upstream on the same leadframe. This helps to guard against any possible damage to the already bonded wires due to strong vibrations of the work-holder during the bonding process.
Another requirement of the index clamp is that both its upper and lower jaws should preferably be actuated during the clamping and de-clamping actions. In earlier generation wire bonding machines, even as a leadframe was passed through the work-holder channels, it always rested on the lower jaw of the index clamp. Only the upper jaw was kept movable in order to effect the clamping action. The difficulties that arise due to such a clamp that only actuates a single jaw will be described in more detail below with reference to the prior art.
A conventional solenoid actuated clamp is generally of the type shown in FIG. 1. It consists of a fixed solenoid 14 and a solenoid plate 15 mounted on a lever 16, which is free to rotate about a pivot 17. When the solenoid 14 is energized with an electric current, it attracts the solenoid plate 15. This rotates the lever 16 as shown by arrow C, about pivot 17. A jaw 18 mounted on the lever 16 thus rotates as shown by arrow D and clamps the substrate 19 against a fixed lower jaw 20. De-clamping of the substrate 19 is effected by turning off the solenoid current, whereat a return spring 21 which was compressed during the clamping action, rotates the lever 16 in a direction opposite to arrow C leading to the opening of jaw 18. The clamp described above is meant for use in the indexing system of a wire bonding machine but can find application in other machines as well. After clamping the substrate 19, the clamp and substrate 19 are moved in a direction perpendicular to the plane of FIG. 1 using either a linear motor or rotary motor with a rotary-to-linear transmission system such as a ball-screw or lead-screw.
There are several drawbacks of this design. First, since the solenoid 14 is a highly non-linear device, its force increases rapidly as the solenoid plate 15 approaches the solenoid 14. This leads to an undesirably high impact on the substrate 19 during clamping. Secondly, since the force falls off rapidly with an increase in the gap between the solenoid 14 and solenoid plate 15, there is a limit to the stroke of the plate displacement and thus a limit to the range of thickness of substrates which can be clamped.
Thirdly, the prior art affords no active control during de-clamping, but has to depend upon the return spring 21. In order to enable quick de-clamping action, the spring force has to be fairly high. This consumes part of the solenoid force, thus leading to a smaller clamping force than would have been possible without the spring 21. Thus, in order to effect a required level of clamping force, the solenoid 14 has to be larger in size.
Fourthly, since only the upper jaw 18 is moved in the clamping process, the lower jaw 20 has to be located precisely such that the substrate 19 rests on the lower jaw 20 even as the substrate 19 is supported by the fixed channel 33. After clamping, the substrate 19 is indexed precisely through a specified distance in a direction perpendicular to the plane of FIG. 1 after which the upper jaw 18 is moved up thus de-clamping the substrate 19. The indexing actuator then returns with the clamp to the initial position for the next indexing stroke. However during the return stroke, the friction between the substrate 19 and the lower jaw 20 on which the former rests, may disturb the precisely-determined position of the substrate 19. To prevent this from happening it is necessary to use another fixed clamp (not shown) which clamps onto the substrate 19 in this precisely-determined position even as the index clamp returns to its initial position for the next indexing stroke. This increases complexity of the system and consumes more space.
It is an object of the invention to seek to provide an improved clamp apparatus that can overcome some of the problems identified with the prior art.
According to a first aspect the invention provides an apparatus for clamping an object, comprising a clamping mechanism and an actuation system to actuate clamping and de-clamping of the clamping mechanism, wherein the actuation system comprises two actuator devices which are operatively coupled whereby to provide an actuation force to the clamping mechanism.
According to a second aspect the invention provides a method for clamping an object, comprising providing a clamping mechanism, providing an actuation system to actuate clamping and de-clamping of the clamping mechanism, wherein the step of providing the actuation system comprises providing two actuator devices which are operatively coupled whereby to provide an actuation force to the clamping mechanism.
It will be convenient to hereinafter describe the invention in greater detail by reference to the accompanying drawings, which illustrate one embodiment of the invention. The particularity of the drawings and the related description is not to be understood as superseding the generality of the broad identification of the invention as defined by the claims.